Photovoltaic (PV) cells are used to convert solar energy (sunlight) into electricity, and are typically implemented either in flat-panel arrangement, or in conjunction with concentrating solar collectors.
Solar Energy Harvesting Requires Inexpensive Large Area Components
Solar energy arrives at the surface of the earth as a relatively dilute form of radiant energy, peaking at approximately 1000 W/m2. Any solar energy harvesting system is therefore required to cover a relatively large area in order to intercept enough sunlight for a meaningful power output. The intercepting area can consist of the energy converting components themselves (e.g., photovoltaic cells in a flat panel module) or consist of optical elements used to direct the intercepted light to a typically smaller converting component (e.g., a higher performance photovoltaic cell in a solar concentrator system). Due to the low price of electricity to which industrialized nations have become accustomed, the key techno-economic challenge and driver is to make the solar energy harvesting system very inexpensive per unit area.
Fabrication processes for photovoltaic cells (PV cells) have benefited from the mature status and sustained progress in semiconductor manufacturing techniques developed for the field of microelectronics. Although it can be expected that process improvements will continue to lower the cost of PV cells into the future, the often cited analogy with Moore's law in microelectronics is only partially appropriate: Moore's law rests heavily on a reduction in surface area per useful unit (e.g. a transistor), while the useful unit in a PV cell is surface area itself. The usefulness of the surface area can be modified in a first example by improving the efficiency of the PV cell, which—being an efficiency metric—naturally has fundamental limits forcing the progress trajectory into an S-curve, and is not the objective of this invention. The usefulness of the surface area can be modified in a second example by optical concentration. It is on of the objectives of this invention to achieve moderate concentration levels (e.g., 10× to 40×, or sometimes higher) with system components that can scale to very low cost and do not incur the system disadvantages typically associated with conventional solar concentrators.
Benefits of Solar Concentration
Historically, a single one of the beneficial aspects of low/medium concentration PV systems dominated the discussion: this dominant aspect was the paradigm of “saving silicon”, which cannot be the only motivation anymore in times of low cost silicon feedstocks now available for PV cell production. Whether flat panel PV cells or PV cells working in somewhat concentrated light environments will form the mainstay of our futures solar energy systems is still heavily debated today. Extrapolations are subject high uncertainty due to the industrial network effects unfolding over time. While flat panel approaches may well win the race to grid parity, there are some important arguments to note in favor of concentrated approaches. These benefits are given here with a bias towards low/medium concentrators and receivers of similar complexity to crystalline silicon PV cells:
(1) A solar harvesting device having only a portion of the surface area consisting of fragile, electrically connected PV cells allows more degrees of freedom in the system design. These can be used to make the system more robust, flexible, easier to ship, partially translucent, building integrated, just to name a few possible directions. An economic degree of freedom is won by the fact, that a low/medium concentrator can make good use of higher efficiency cells (e.g. high efficiency silicon or similar), that might be temporarily or systematically not quite competitive for flat panel application under 1× sun.
(2) A large fraction of the capital expenditure of PV module manufacturing goes towards the fabrication machinery for PV cell manufacturing. Producing more total PV module area per year normally requires the installation of proportionally more machinery. This can limit the growth of such a technology and business domain, as the capital for expansion often is the limiting resource. Organic growth from reinvesting profits may be too slow for staying on top of competitors in the market or climate goals in the world. A concentration system can mitigate such capital imposed limits and allow faster scale-up in terms of GWp/year, since the electrical productivity of each cell leaving the (capacity limited) production line is increased by the flux concentration factor F=Cg*η (where η is the optical efficiency of the concentrator). Of course, this strategy only holds if the capital cost to obtain fabrication capacity for the optical concentrator is lower on a per Watt basis than the PV cell fabrication facility (fab). This is particularly true for optical concentration elements that can be manufactured on existing machinery. As explained below, systems described in this invention can be manufactured e.g. on fabrication lines similar to those used for building windows (“insulating glass units”, IGUs). The optical focusing elements, particularly lenses, can be straightforwardly realized in multiple ways. As a first exemplary option, the low optical demands on the (non-imaging) topography allow to use simple “textured glasses” (e.g. verified with “Pilkington Cross Reed 0.5 inch” glass), which are produced at low cost for fenestration products. As a second exemplary option, the optical focusing elements can manufactured on film or foil manufacturing equipment, such as the microoptical film fabrication equipment that exists today for display backlighting films. These plants have an annual capacity in the hundreds of square kilometers per year. The ability to rapidly scale up production will become a particularly prominent competitive differentiator, as soon as solar energy first undercuts the costs of established fossil fuel based generation.
(3) Concentrator photovoltaics can drastically reduce the embodied energy in the solar energy harvesting system per peak Watt installed when compared to flat panel PV cells. This shortens the energy payback time of the system and similarly the “energy returned on energy invested”.
(4) Returning to the initial point made above, a low/medium solar concentrator with good manufacturability can reduce the cost per installed W compared to flat panel systems, if disadvantages of prior art concentrators (such as tracking requirements) are avoided.
Passive Optical Concentrators in the Prior Art
Prior art solar concentrators utilize optics (e.g., reflectors, lenses, etc.) to focus sunlight onto a relatively small PV cell. This can be motivated by direct cost savings (e.g., when the area specific cost of the optics is lower than the cost of the PV cell), and/or by the desire for higher system efficiencies (e.g., by allowing to use high performance PV cells that are only available and economic in small areas).
In prior art passive optical systems, concentration typically leads to the requirement of mechanical tracking. The roots for this causation are of fundamental physical nature, and can be outlined as follows: Concentration in the spatial domain comes at the expense of an expansion in the angular domain. This is mandated by principles of conservation of phase space (i.e., Etendue). The concentration sought from a solar concentrator is a concentration in the spatial domain: The energy intercepted at a large area aperture is coupled to a small area receiver (photovoltaic or thermal) having a surface area that is smaller by a factor Cg. This causes the solid angle subtended by the incoming radiation to expand by the incoming radiation to expand by approximately the same factor (modified by the refractive index contrast and projection direction) before it reaches the smaller receiver. However, the solid angle from which a receiver can accept light is typically limited to 2π2 (hemispherical space) or in some cases to the absolute limit of the full sphere at 4π2. This limits the solid angle from which a concentrator can efficiently accept incoming radiation at its input. However, even direct sunlight originates over the course of year from within a significant portion of the sky hemisphere. The acceptance solid angle starts to become restricted to a solid angle zone narrower than this even for very low spatial concentration factors Cg, e.g. 3×. This can be improved upon by optimizing for the particular angular intensity distribution, but passive static systems beyond 10× concentration are impractical on earth.
It should be noted that the direct sunlight itself subtends only a very small solid angle at any given time. Based on this, prior art systems are able to efficiently reach higher concentration factors by going from static (untracked) systems to tracked concentrators. These tracking systems keep the relative angular position between the sun and the concentrator substantially constant in one or two of the angular dimensions; typically by mechanical movement of the systems. Mechanical tracking systems add installation cost, maintenance cost, reliability concerns, windloading problems and other disadvantages to the system. A system that achieves higher concentration factors than static concentrators without mechanical tracking is therefore highly desirable.
Luminescent Solar Concentrators in the Prior Art
A Luminescent Solar Concentrator (LSC) allows concentration without tracking of both diffuse and direct radiation and have been described in the prior art. LSCs overcome the single wavelength Etendue limits that constrain passive optical concentrators by subjecting each photon to a downward shift in energy (towards longer wavelength), e.g. via a fluorescence process. The photon energy difference is required for compliance with the governing thermodynamic principles and enables concentration factors well beyond the domain to which static concentrators are limited otherwise.
Luminescent Concentrators (LSCs) are designed to achieve a result similar to the present invention (concentration without tracking), but via a very different route: To capture the incident light, they rely on luminescent materials. The present invention does not rely on luminescent materials, but instead uses a self adaptive strategy with materials that are optically passive. After the emitted light (or scattered light respectively) is captured by total internal reflection in the lightguide, the same broad range of options to outcouple and utilize it are applicable again.
Particularly for devices integrated into building envelopes, there are two important aspects that favor the approach of the present invention over LSCs:
(1) This approach selectively couples the direct component of sunlight, while letting the diffuse daylight component pass. Thus, the two components can be handled separately (e.g. direct light used for pv electricity, diffuse light used for room lighting). The absorption in an LSC doesn't differentiate between the two angularly distinct light components, and therefore does not offer the degree of freedom offered here.(2) While LSCs can in principle be made to exhibit a color-neutral spectrum in the residual light transmitted and their appearance, this is practically quite challenging to achieve. Since the present invention does not rely on any mechanism that (like luminescence) affects the wavelength of a captured photon, color neutrality is achieved without special consideration. This is desirable for building integrated applications.Waveguide-Type Solar Concentrators
FIG. 13(A) shows a recently introduced solar energy capturing system 60 proposed by Karp et al. (see WO20/0/033859, “SYSTEM AND METHOD FOR SOLAR ENERGY CAPTURE AND RELATED METHOD OF MANUFACTURING” that includes a waveguide (lightguide) component 61. Contrasting to e.g. luminescent solar collectors, system 60 employs a waveguide component 61 from a clear, non-luminescent material (e.g., glass or suitable plastic, such as PMMA or PC), and utilizing a lens array 70 having a plurality of lenses 75 to focus sunlight SL onto an array of prism/mirrored facets 68 arranged along at least one surface (e.g., lower surface 64) of waveguide component 61. That is, as illustrated in FIG. 13(A), incident sunlight SL directed through upper surface 71 of array 70 is focused by lenses 75, and the focused sunlight FSL is directed through upper surface 62 of waveguide component 61 onto prism/mirrored facets 68. Waveguide component 61 is formed such that reflected sunlight RSL, which is the focused sunlight FSL that is reflected back toward upper surface 62 by prism/mirrored facets 68, is directed to remain in component 61 by internal reflection from the waveguide surfaces 62 and 64. The system further includes at least one photovoltaic cell 50 positioned so as to receive at least a portion of the reflected sunlight RSL.
FIG. 13(B) illustrates a problem associated with the solar energy capturing system 60. In particular, when incident sunlight SL is directed at a non-optimal angle onto lens array 70, the focused sunlight FSL is directed away from prism/mirrored facets 68, thereby preventing reflected sunlight from being captured within waveguide component 61. One approach for avoiding this problem is to provide a tracking system that repositions system 60 as the sun moves across the sky, but this solution suffers the problems described above with the cost and complication associated with the tracking system. It should be noted that mechanically tracked PV systems are very difficult to integrate into building envelopes, due to the motion requirements, aesthetics and wind load considerations. Tracking with 2 degrees of freedom (as is a requirement with the prior art system 60) is particularly challenging, since at least the second degree of freedom will cause the device to rotate out of plane. In the presence of wind, it is costly to support a large area element that is out of plane, instead of being flush with the structure.
Another approach would be to increase the size and/or modify the shape of prism/mirrored facets 68 such that the focused sunlight remains directed onto facets 68 throughout the day. However, this approach requires covering a significant portion of the waveguide surfaces with facet structures, which impedes internal reflection and thus reduces the amount of reflected sunlight that is successfully guided along waveguide component 61 to photovoltaic cells 50. Achieving acceptance angles large enough to avoid tracking requirement, while providing sufficient concentration levels and optical efficiency is not feasible in this way.
What is needed is a solar energy harvesting device that provides the advantages of a lightguide-type (waveguide-type) solar concentrator, but avoids the alignment/tracking requirement associated with existing prior art devices.